The long term goal of this project is to gain insight into the nature and role of human B1 cells in health and disease. The specific aims of the present project are to test the hypothesis that B1 cells are the source of pathogenic autoantibodies in systemic lupus erythematosis (SLE, lupus), and to identify the progenitor for B1 cells in order to provide a foundation for potential novel therapies against lupus and related autoimmune dyscrasias. Lupus is a devastating and sometimes fatal multi-system affliction in which normal tissues are attacked by a dysfunctioning immune system. Autoreactive antibodies are an indelible feature of lupus and are thought to play a pathogenic role. The source of these autoreactive antibodies remains uncertain. Current paradigms have not included a role for B1 cells because: 1) the definition of human B1 cells has been unclear; and, 2) clinically significant autoantibodies are typically IgG and somatically mutated (whereas well characterized mouse B1 cells generate mostly IgM with little mutation that fulfills a homeostatic role by speeding elimination of apoptotic cell debris). However, two new findings bring B1 cells into the picture. Firstly, we identified the phenotype (CD20+CD27+CD43+) for human B1-like cells that share characteristics with murine B1 cells. Secondly, we determined that antibody produced by human B1- like cells is often IgG and typically somatically mutated. These new results suggest the hypothesis that abnormally functioning B1 cells are the source of pathological autoantibodies in lupus and that the ratio of IgG to IgM may determine relative destructiveness and consequent loss of tolerance. As a key corollary, in order to modulate B1 cell activity, it is necessary to identify the progenitor for these cells and to understand the means to influence their development. To address these issues we propose that: 1. We will identify the progenitor for human B1 cells. 1A. We will identify the human B1 cell progenitor by adoptively transferring candidate populations to immunodeficient NSG mice and monitoring B1 cell development. 1B. We will test enhancements to B1 cell development by evaluating cytokine ligands during in vitro culture. 2. We will examine the role of B1 cells in generating autoantibodies. 2A.We will determine the autoreactivity of antibodies derived from B1 cells and other B cell populations by testing expression-cloned VH4-34 antibodies. 2B. We will evaluate the secretion of autoantibodies by B1 and other B cells by determining the frequency/amount of IgG/IgM after stimulation, comparing lupus and control B cells, to test our hypothesis that lupus B1 cells specifically generate overwhelming amounts of IgG autoantibody. Much is known about mouse B1 cells whereas little is known about human B1-like cells. The results of these studies will greatly increase knowledge regarding the nature and development of human B1 cells; will test a novel hypothesis linking B1 cells and pathological autoantibodies; and, will provide a foundation for future efforts to therapeutically modify B1 cell activity.